The present invention relates to a sintered hard metal, i.e., a cemented carbide, body having an essentially increased wear resistance obtained by boronizing the cemented carbide.
Sintered hard metal bodies consisting of at least one carbide and a binder metal have a versatile application in industry. They are used in cutting and for details exposed to heavy wear. A great resistance to wear is, as understood, a very important property in many of these applications.
It is known that diffusion zones of boron or thin layers of borides, carbides or nitrides on the surface of a hard metal body can considerably increase the surface hardness of a detail. Also, particles of diamond or cubic boron nitride, possibly together with a binder phase consisting of Co, Ni, Ti or other metals, have been used as layers on hard metal or by themselves as homogenous bodies.
The above-mentioned bodies have been developed particularly for applications demanding high wear resistance. In these applications the applied layers often flake (or splinter if they are thick) because of mechanical strains. Furthermore, the improved high temperature properties (as chemical stability, oxidation properties) of the boride-, carbide- or nitride-layers, compared to untreated hard metal, cannot be exploited if the working temperature is below about 600.degree. C., which often is the case in applications outside the field of chip-forming metal cutting. Products containing diamond or cubic boron nitride in the form of layers or as solid bodies are not often used because of their expensive cost.
In hard metal products, known up to now, having diffused zones of boron, the boronized zones have been of unsatisfactory quality and thickness. Often, the surface zone has been so thin that it was prematurely broken through because of wear. This means that the increased wear resistance of the surface zone is not fully exploited. Furthermore, thick surface zones mean that working operations as polishing, grinding, lapping, etc., can be used after the boronizing.
Processes developed for boronizing steel have normally been used for boronizing cemented carbide, which means that the process has not been as well controlled as needed for boronizing multiphase materials like cemented carbide. In addition, the cemented carbide substrate has not been a particular grade optimum for boronizing.
Materials containing boron and being used as boron donors in boronizing are available in all three physical states. For steel, good results have been obtained by packing methods using powdered boron compounds or phases together with, e.g., aluminum oxide powder and possibly "activators." Very little progress has been obtained, however, with methods exploiting liquid phases (salt melts), including the use of electrolysis. Thus, the high viscosity of such phases means that the process conditions have been difficult to control.
It has also not been possible to use gas boronizing, the method offering the best means of control in the optimum way. Thus, it has not been possible to avoid a too vigorous boronizing which means, among other things, a considerable embrittlement of the boronized surface zone and the presence of free boron on the hard metal surface. Also, zones which are too thin have been obtained. The reason is that only two groups of gaseous boron compounds are known, the boranes and the borontrihalides. The boranes are very poisonous and expensive while the use of borontrihalides has been considered as leading to a vigorous corrosion of the substrate.